Pertussis vaccine's effectiveness wanes after final dose, study finds

If you live in California, chances are you remember the pertussis outbreak of 2010. That year, Los Angeles alone saw nearly 1,000 cases of the illness, also known as whooping cough, and the state as a whole saw around 9,000. This year, the nation is on pace to have its largest whooping cough caseload since 1959.

Now a new study suggests a troubling reason why such outbreaks may be becoming more common: The DTaP vaccine appears to wane in effectiveness after the fifth and final recommended dose, which is generally received around the age of 5.

Whooping cough, caused by a bacterium, can be a nasty respiratory illness that causes bouts of coughing and its namesake whooping noise, which is made by sufferers when they take deep breaths after a series of coughs.

In the new study, published this week in the New England Journal of Medicine, researchers at the Kaiser Permanente Vaccine Study Center in Oakland analyzed data from 277 children who tested positive for pertussis between 2006 and 2011, and compared them with over 3,000 children who tested negative, as well as an additional 6,000 matched controls.

Compared with the controls, the researchers found that children who had pertussis had received their fifth dose earlier. And the group concluded that for every year that goes by after the fifth dose, the vaccine becomes 42% less effective.

This means if the vaccine is 95% effective to begin with -- the group's most optimistic estimate -- that effectiveness drops to 79% after five years. The federal Centers for Disease Control and Prevention, in its own study, has arrived at similar numbers.

The study's authors say the problem may rest with a new vaccine that was introduced in the early 1990s. It was considered better than the vaccine it replaced because it had fewer side effects.

The researchers have taken pains to point out that the vaccine remains a child's best protection against pertussis. But they also argue that more work needs to be done to improve the vaccine, writing that "our findings highlight the need to develop new pertussis-containing vaccines that will provide long-lasting immunity."